1. Field of the Invention
The invention relates to a semiconductor device and a method of manufacturing the same, particularly, a semiconductor device having electrodes on either side of a semiconductor substrate and a method of manufacturing the same.
2. Description of the Related Art
A power transistor is widely used as a switching element for power supply. A vertical MOS transistor in which a source-drain current flows in a vertical direction relative to a front surface of a semiconductor substrate is known as a kind of power transistor.
A vertical MOS transistor will be described referring to figures. FIG. 13A is a plan view of the vertical MOS transistor on the front surface side, and FIG. 13B is a cross-sectional view of FIG. 13A along line Y-Y.
A source electrode 101 and a gate electrode 104 are formed as front surface electrodes on the front surface of a semiconductor substrate 100, being connected to a source region and a gate (not shown) formed on the front surface. Bump electrodes 102, 105 for electrically connecting the source electrode 101 and the gate electrode 104 to a circuit board (not shown) (e.g. a printed board) are formed on the source electrode 101 and the gate electrode 104. The source electrode 101 and the gate electrode 104 are covered by a protection film 103, with the bump electrodes 102, 105 being exposed.
On the other hand, a drain electrode 106 as a back surface electrode is formed on the back surface of the semiconductor substrate 100, being connected to a drain region of the semiconductor substrate 100.
Such a vertical MOS transistor is described in Japanese Patent Application Publication No. 2008-66694.
However, the semiconductor substrate 100 formed with the vertical MOS transistor described above may warp when it is used. This is likely because stress applied to the interface between the semiconductor substrate 100 and the front surface electrodes and stress applied to the interface between the semiconductor substrate 100 and the back surface electrode are different when the front surface electrodes and the back surface electrode have different coefficients of linear expansion. The warping of the semiconductor substrate 100 may cause the protection film 103 or the bump electrodes 102, 105 to peel off and further the front surface electrodes or the back surface electrode to peel off. Since this warping of the semiconductor substrate 100 varies depending on changes in temperature of an environment, the repetition of the temperature change likely causes the protection film 103 and so on to peel off easily.
Furthermore, there is a need for enhancement of the current drive ability of the vertical MOS transistor described above. In order to enhance the current drive ability, according to the inventors' examination, an effective approach is to increase the thicknesses of the front surface electrodes in the vertical direction. This is because the bump electrodes 102, 105 are formed on part of the front surface electrodes extending over the front surface of the semiconductor substrate 100 as shown in FIG. 13A and the amount of electric current components that flow in the front surface electrodes in the horizontal direction relative to the semiconductor substrate 100 is large. Increasing the thickness of the back surface electrode in the vertical direction is also an effective approach.
For this reason, a plating method is suitable as a method of manufacturing the front surface electrodes and the back surface electrode in order to increase the thicknesses of the front surface electrodes and the back surface electrode in the vertical direction.
In detail, a resist layer having openings in the regions for forming the front surface electrodes is formed on the front surface of the semiconductor substrate 100 first, and the front surface electrodes are formed in the openings by a plating method using the resist layer as a plating mask. The back surface electrode is formed on the whole back surface of the semiconductor substrate 100.
However, in the process of manufacturing the vertical MOS transistor, when the semiconductor substrate 100 is thinned, the semiconductor substrate 100 may warp due to the process of forming the back surface electrode 106 or a heat treatment process to cause the resist layer to have a formation defect such as peeling. This formation defect of the resist layer naturally causes the front surface electrode a formation defect.
Therefore, the invention is directed to forming a thick front surface electrode by a plating method to enhance the current drive ability of a transistor, and preventing a formation defect of a resist layer due to the warping of the semiconductor substrate 100.